Major emphasis is directed to an understanding of the organization and control of expression of the rosy locus in Drosophila melanogaster. Specific projects include: (1) Through the use of cloned probes, a description of the time course and tissue distribution during development of rosy expression in normal and selected variant genotypes at the level of XDH specific transcripts and message. (2) Selective mutagenesis experiments are proposed to make and characterize overproducer control variants. (3) We propose experiments to identify trans-acting, transcriptional regulatory molecules of the rosy locus, to make mutations of the gene or genes responsible for their production, and to characterize the regulatory macromolecular interactions. (4) We propose to construct a synthetic tandem and dispersed repeat system to examine the role of conversion in maintenance of homogeneity, and to examine factors involved in stability of tandem repeat systems. Finally, collaborative studies on nonsense mutants and nonsense suppressors and on chromatin organization are discussed.